Gas turbines generally operate by combusting a fuel and air mixture in one or more combustors to create a high-energy combustion gas that passes through a turbine, thereby causing a turbine rotor shaft to rotate. The rotational energy of the rotor shaft may be converted to electrical energy via a generator coupled to the rotor shaft. Each combustor generally includes fuel nozzles that provide for delivery of the fuel and air upstream of a combustion chamber, using premixing of the fuel and air as a means to keep nitrogen oxide (NOx) emissions low.
Gaseous fuels, such as natural gas, often are employed as a combustible fluid in gas turbine engines used to generate electricity. In some instances, it may be desirable for the combustion system to be able to combust liquid fuels, such as distillate oil, either simultaneously with or instead of gaseous fuel. A configuration with both gas and liquid fuel capability is called a “dual-fuel” combustion system.
Cooling techniques that prevent thermal breakdown of the liquid fuel and the formation of coke in/on dual-fuel fuel nozzles that supply liquid fuel to the combustion chamber must be considered when designing these types of fuel nozzles. If coke (i.e., carbon formation) is allowed to form, it can cause blockages within the fuel system. Typically, the liquid fuel injector is surrounded by air at elevated temperatures, which are significantly above the temperatures at which coke may be expected to form. To maintain acceptable wetted wall temperatures within the fuel delivery tubes, the liquid fuel itself is often used as a heat sink. However, if the fuel is not moving at a sufficient flow rate, the coke formation temperature may be reached. Likewise, if the air flow surrounding or contacting the liquid fuel delivery tubes is moving too quickly and transferring too much heat to the liquid fuel delivery tubes, the coke formation temperature may be reached.